1. Field of the Invention
This invention relates generally to the field of digital signal processing and more particularly to an equalizing and filtering system suitable for processing eight symbol vestigial sideband (VSB) High Definition Television (HDTV) signals.
2. Background of the Invention
The Advanced Television Systems Committee (ATSC) standard for HDTV in the United States specifies an eight bit (eight levels per symbol) vestigial sideband (VSB) transmission system. A block diagram of a typical ATSC compatible HDTV receiver 21 is depicted in FIG. 1. The specifications for such an HDTV receiver can be found in the publication Digital Television Standards for HDTV Transmission of Apr. 12, 1995 prepared by the ATSC. The received VSB signal is represented by a one dimensional data symbol constellation in which only one axis contains quantized data to be recovered by the receiver. A DC compensation unit 26 uses an adaptive tracking circuit to remove from the demodulated VSB signal a DC offset component to the pilot signal component. Field sync detector 28 recovers the data field sync component by comparing every received data segment with an ideal field reference signal stored in memory in the receiver. In addition to field synchronization, the field sync signal provides a training signal for the channel equalizer 34.
In the ATSC VSB receiving system, NTSC interference detection and rejection are performed by unit 16. When strong co-channel NTSC interference is detected by unit 16, a comb filter is activated to process the signal prior to reaching the adaptive channel equalizer 34. The adaptive equalizer 34 equalizes both channel amplitude and phase distortion. As depicted in FIG. 2, the equalizer structure that is most commonly utilized includes a Feed Forward Equalizer (FFE) that is used in conjunction with a Decision Feedback Equalizer (DFE). The FFE section is used to equalize preghost signals and channel zeroes that reside outside of the unit circle of the data symbol constellation. The DFE section depicted in FIG. 3 is used to equalize post ghost signals.
A method that is commonly used to train the equalizer 34 is to apply the Blind Equalization technique until the equalizer has reached some predetermined degree of convergence, at which time the training method is switched to the Decision Directed Equalization technique. The most common form of Blind Equalization is the well known Constant Modulus Algorithm (CMA). Using the CMA, the FFE and DFE coefficients are updated using the following equations:
Cn+1 = Cn + αF * e(n) * XFn(FFE update)Dn+1 = Dn + αD * e(n) * XDn(DFE update)
where:                Cn=FFE coefficient vector        Dn=DFE coefficient vector        XFn=Data vector into the FFE        XDn=Data vector into the DFE        e(n)=Blind equalization error.        
The blind equalization error is calculated according to the equation:e(n)=y0(n)*(|y0(n)|2−R2)
where:                y0(n)=Equalizer output (FFE+DFE output)        R2=Ring value squared         =E(s4(n)/E(s2(n))        s(n)=eight VSB constellation points.        
The comb filter 2 has a transfer function of Hc(z)=1−z−12. The comb filter attenuates the picture and sound carriers of the adjacent NTSC channel. The new signal that the equalizer 34 must then process is the original channel convolved with the comb filter. When the comb filter is inserted prior to the equalizer 34 and the DFE taps are adapted using the CMA, the equalizer 34 produces an eight level output. In effect, the equalizer 34 equalizes both the channel and the comb filter signals. However, the goal is cause the equalizer 34 to produce a fifteen level output by equalizing only the channel signal and not equalizing the comb filter signal.
U.S. patent application Ser. No. 10/103,055, filed on Apr. 16, 2002, describes a method in which the equalizer 34 produces a fifteen symbol output when a comb filter is inserted to reject the NTSC interference. The described method solves the problem of obtaining a fifteen level output by zeroing tap twelve, dfe(12) of the Decision Feedback Equalizer 34. The limitation of this method is that the channel amplitude at symbol twelve, c12, must be small, typically on the order of less than −20 dB.